A top White House official says the continued flow of oil into the Gulf of Mexico is probably "the biggest environmental disaster" that the United States has ever experienced.

Carol Browner, a White House adviser on energy and climate change, says the oil may continue to leak until August and that the United States is, as she said, "prepared for the worst."

President Barack Obama earlier called the spill "enraging."

An estimated 12,000 to 19,000 barrels of crude are said to be leaking into the Gulf each day from a well pipe that ruptured when the Deepwater Horizon oil rig exploded and then sank two days later on April 22.

Efforts by BP to contain the damage and stop the spill have failed so far.

Browner today said the U.S. government has ordered BP to drill two relief wells in case one doesn't work.